Supression of laser breakdown by pulsed nonequilibrium NS discharge

Remote control of plasmas induced by laser radiation in the atmosphere is one of the challenging issues of free space communication, long-distance energy transmission, remote sensing of the atmosphere, and standoff detection of trace gases and bio-threat species. Sequences of laser pulses offer an advantageous tool providing access to the control of air-plasma dynamics and optical interactions. Recently, in [1] was proposed to use a dual femtosecond/nanosecond laser pulse to enhance the energy density locally deposited in laser-induced plasma in the atmosphere. Experiments [2] have demonstrated that the lifetime of plasma channels generated through filamentation of femtosecond laser pulses can be increased by applying a delayed nanosecond pulse. In this paper we have shown that preionization of gas does not always lead to better conditions for the development of a laser spark. High temperature of electrons and their high concentration in the afterglow of the pulsed nanosecond discharge may lead to an increased rate of energy exchange between the region of the focus of the laser beam and the surrounding plasma. Effective diffusion cooling of the electron's ensemble reduces the ionization rate and makes it impossible to develop a laser breakdown at low gas pressures. It should be noted that this case is completely different from the laser double pulse technique. In the case of the preionisation by multiphoton processes in a focused laser beam the ionized region has a small size. Due to the presence of significant ambipolar field electron diffusion and transfer of energy from this preionized region is severely hampered and the second laser pulse heats the electron gas effectively.